The use of fluidized cracking catalyst (FCC) containing zeolite Y in order to crack gas oils into various products, including gasoline and fuel oil, is widely practiced commercially. In catalytic cracking an inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In a fluidized catalytic cracking (FCC) system, a stream of hydrocarbon feed is contacted with fluidized catalyst particles in the hydrocarbon cracking zone, or reactor, at a temperature of about 425.degree.-600.degree. C., usually 460.degree.-560.degree. C. The reactions of hydrocarbons at the elevated operating temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid products are separated from the deactivated, spent catalyst and are withdrawn from the cracking reactor. The spent catalyst particles, containing a substantial concentration of coke, are stripped of volatiles, usually by means of steam, and are then passed to the catalyst regenerator. In the regeneration zone, the coked catalyst is contacted with a predetermined amount of oxygen-containing gas. A desired portion of the coke is burned off the catalyst, simultaneously restoring catalyst activity and heating the catalyst to a higher temperature required for use in the cracking zone, e.g., 540.degree.-815.degree. C., usually 590.degree.-730.degree. C. Flue gas is formed by combustion of the coke in the regenerator. The flue gas may be treated for removal of particulates and conversion of carbon monoxide, after which it is normally discharged into the atmosphere.
The activity of a cracking catalyst is an important parameter in catalytic cracking operations. A standard measure of the activity of a cracking catalyst is the degree of conversion which can be obtained using the catalyst in a cracking operation. The degree of conversion may be defined as the volume percent of fresh hydrocarbon feed having a normal boiling point of at least 221.degree. C. which is changed to gasoline and lighter hydrocarbon products during the cracking conversion step, where the end point of gasoline for the purpose of determining conversion may be defined as 220.degree. C. In addition to being a measure of catalyst activity, the conversion obtained with the catalyst can also be used as a measure of the severity of a cracking operation, so the activity of a catalyst is determined by conversion at a predetermined, standard set of operating conditions. At a given set of operating conditions, a more active catalyst gives a greater conversion than does a less active catalyst. Increased conversion is a desirable attribute in a cracking catalyst. Higher conversion allows flexible operation of a cracking unit. For example, when conversion is raised, feed throughput can be increased, or a higher degree of feed conversion can be maintained with a constant feed throughput.
Because of catalyst attrition, imperfect gas-solids separation, etc., catalyst in a cracking unit is continuously being lost from the circulating inventory. The desired catalyst inventory level is conventionally maintained by constant addition of fresh, or rejuvenated, make-up catalyst. Accordingly, the catalyst inventory in a given FCC unit is a mixture of particles which have been in use for widely varying periods, and which contain varying amounts of coke and contaminants such as metals. The mixture of catalyst particles forming the inventory during normal unit operation is referred to as "equilibrium catalyst". The activity of catalyst used in commercial cracking units is generally measured on the basis of the average activity of the equilibrium catalyst. Fresh cracking catalyst is known to possess a much higher activity than equilibrium catalyst or catalyst which has been used in a cracking operation for a relatively short time.
As can be appreciated, the disposal of equilibrium catalysts presents serious environmental problems since they contain appreciable amounts of heavy metals such as nickel and vanadium and since large amounts must be handled.
Various proposals have been presented for dealing with such spent catalyst. The vast majority of proposals are concerned with preserving the crystallinity of zeolite Y while rejuvenating or reactivating the same by various treatments. Other proposals involve converting zeolite Y into another zeolite such as zeolite A.
U.S. Pat. No. 4,784,980, the entire disclosure of which is incorporated herein by reference, deals with the latter approach while also disclosing prior art techniques directed towards the former approach.
British Patent 1,342,977 discloses the conversion of equilibrated cracking catalysts into zeolite Y.